The present invention relates generally to adhesive coatings, and, more particularly, to adhesive coatings for airbags utilizing silicone polymer blends.
The use of airbags as safety features in motor vehicles is well known. Airbags are installed on the driver and passenger side of automobiles and, in the event of a collision, are rapidly inflated with gas to act as a barrier between the driver or passenger and the steering wheel or dashboard of the automobile.
There exist three primary types of airbags, each for different end uses. Driver side airbags are generally mounted within steering columns and exhibit relatively low air retention in order to act more as a cushion for the driver upon impact. Passenger-side airbags also comprise relatively high air permeability fabrics that permit release of gas either by percolation of the gas through the fabric or through vents integrated therein. Both of these types of airbags (composed of multiple fabric panels) are designed to protect persons in sudden collisions and generally burst out of packing modules from either a steering column or dashboard. Side curtain airbags, however, have been designed primarily to protect passengers during rollover crashes by retaining the inflation state for a long duration and generally unroll from packing containers stored within the roofline along the side windows of an automobile. Side curtain airbags therefore not only provide cushioning effects but also provide protection from broken glass and other debris. Therefore, it is imperative that side curtain airbags, as noted above, retain large amounts of gas, as well as high gas pressures, to remain inflated throughout the longer time periods of the entire potential rollover situation. Accordingly, depending on the particular end use of the airbag, various features must be included in the structure of the airbag.
Coatings have been applied to fabrics, intended for use in automotive airbags, to resist the unwanted permeation of air through the fabric and, to a lesser extent, to protect the fabric from the hot gases used to inflate the bags. Polychloroprene was the polymer of choice in the early development of this product, but the desire to decrease the folded size of the completed airbag, and the tendency of polychloroprene to degrade, with exposure to heat, and release the components of hydrochloric acid (thereby potentially degrading the fabric component as well as releasing hazardous chemicals), has led to the almost universal acceptance of silicone (polydimethylsiloxane or similar materials) as a more suitable coating. Silicone polymers have excellent thermal properties.
However, silicone polymers have relatively high permeability to gases, when compared to many other elastomers. This feature has not been a matter of concern in coatings used for driver side airbags, as the retention time requirements are very small. However, through the advent of side curtains, which require higher air retention, the retention time has become a greater concern.
Furthermore, the utilization of such silicone polymers has, in the past, come at a price. The costs associated with such silicone compounds are generally quite high, particularly the costs required to provide sufficient coverage of target fabrics while best ensuring low permeability will continue as long as necessary. Furthermore, although lower levels of other types of coatings (thermoplastics and thermosets, such as polyurethanes, for example) have been utilized for such a purpose, there are general add-on amounts that, to date, are required to provide needed long-term inflation gas retention rates for target silicone-coated airbag cushions. As stated above, silicone coating materials are generally preferred over other polymer types due to their ability to withstand varied environmental and storage conditions over long duration.
Yarn shifting has also proven to be a significant problem for airbags. When a sewn seam is put under stress, a naturally lubricating silicone coating may allow the yarns from which the fabric is constructed to shift. This shifting can lead to leakage of the inflating gas through the new pores formed from the shifting yarns, or, in drastic cases, cause the seam to fail. Since the airbag must retain its integrity during a collision event, in order to sufficiently protect the driver or passenger, there is a great need to provide coatings which provide both effective permeability characteristics and sufficient restriction of yarn shifting for the airbag to function properly, if and when necessary. Again, such a coating material is preferably silicone in nature for storage purposes. Therefore, a need exists to provide such beneficial characteristics at lower cost and/or lower add-on levels through an airbag coating that provides low permeability, resistance to yarn shifting and age resistance over long periods of storage.
As another issue, it has recently been found that more efficient side curtain airbag cushions may be produced as one-piece woven (preferably Jacquard woven) articles. Interestingly, the requirements for effective coatings for such one-piece woven airbags are significantly different from those needed for standard driver or passenger side airbags. A one-piece Jacquard (for example) airbag cushion is more economical to produce due to the elimination of the need to first cut fabric portions from coated webs and subsequently sew them together. The distinct disadvantage of this system is that the target bag must be coated on the outside during production, (as opposed to a sewn bag in which the coated face is normally placed within the interior of the air bag). When the Jacquard woven bag is then deployed, inflation pressures may be transmitted through the fabric to the coating, applying a potentially delaminating force to that coating. If the adhesion of the coating to the fabric is strong, then the diffusion forces are localized and, depending upon the strength of the coating film, may lead to a rupture of the film itself, whereupon the inflation gases can easily escape. If the airbag is intended as a side curtain, such inflation gas loss would severely reduce the effectiveness of the inflated airbag and jeopardize its ability to protect during a long duration rollover scenario. On the other hand, if the adhesion of the coating is less strong, then the diffusing force can be dissipated by localized delamination of the film without rupture thereof. This would typically result in a blister (known in the airbag coating industry as an aneurysm) wherein the inflating gases can be retained, but the appearance of the bag, is objectionable, regardless of the fact that the bag itself most likely retains the inflation gases: therein. Thus, coatings for such one-piece woven airbags must take into account this dichotomy and balance the adhesion of the coating with the retention of the inflating gases. To date, such a balance of considerations in developing proper airbag coatings, particularly for one-piece woven airbag cushions, has not been exercised.
Thus, it is highly desired to utilize a trustworthy, high inflation gas retention, coating for low permeability airbag cushions, particularly with relatively low costs involved in providing such benefits. Further, the inclusion of a host of other features for airbag coatings may also be desirable depending on the end use of the airbag, including enhanced adhesion of the coating to the bag, greater tear, tensile, and flexural strengths, increased elasticity, biocide and antimicrobial capabilities, and inability to burn.
It has been shown that by forming what are called “interpenetrating polymer networks” (IPNs), the behavior of silicone polymers can be modified. IPNs are a special class of polymer blends in which the polymers exist in networks that are formed when at least one of the polymers is synthesized or cross-lined in the presence of the other. Classical or true IPNs occur when all of polymer species within a blend form chemical crosslinks. More recently, two other types of IPNs have been developed. Apparent IPNs are based on combinations of physically cross-linked polymers. Semi-IPNs have also been formed, which include a combination of cross-linkable and nonreactive linear polymers. In a semi-IPN of polysiloxane (silicone), the silicone component cross-links to itself in the presence of another nonreactive polymer. When this occurs, the nonreactive polymer becomes trapped in the silicone network, and, as a result, imparts additional properties included in the nonreactive polymer to silicone.
Accordingly, the formation of IPNs based on blends of silicone polymers with other polymers having desirable features and characteristics not found in the silicone alone has the potential of greatly enhancing an airbag coating. The extent of formation of these IPNs, however, is limited by the compatibility of the thermodynamic properties of the additional polymers to those of silicone. The most effectively formed IPNs are those in which the constituents are thermodynamically compatible. Whereas previous attempts—as are described in U.S. Pat. Nos. 6,348,543; 6,468,929; and 6,545,092—were successful in forming silicone blends to improve the properties of silicone, such as resistance to seam combing in airbags, these attempts did not address the thermodynamic properties of the silicone polymers compared to those polymers that are blended with silicone to form an IPN. Because it may be desirable to achieve a silicone polymer blend having other properties, such as reduced air permeability and lower cost, there exists a need for identifying a parameter whereby one can predict the likelihood of success in combining polymers having particularly attractive characteristics with silicone polymers to form IPNs.